Added MKR1000 board to list to define the dtostrf function

Adding the MKR1000 board to the list of boards for which the dtostrf
function should be defined here (for lack of a better way to determine
if the function is available elsewhere).

Adafruit_MQTT.cpp

@@ -21,7 +21,7 @@
 // SOFTWARE.
 #include "Adafruit_MQTT.h"
 
-#ifndef dtostrf
+#if defined(ARDUINO_SAMD_ZERO) || defined(ARDUINO_SAMD_MKR1000)
 static char *dtostrf (double val, signed char width, unsigned char prec, char *sout) {
   char fmt[20];
   sprintf(fmt, "%%%d.%df", width, prec);